The past few years have brought about tremendous changes in the modern home, and especially, in appliances and other equipment designed for home use. For example, advances in personal computing technologies have produced faster, more complex, more powerful, more user-friendly, and less expensive personal computers (PCs) than previous models. Consequently, PCs have proliferated and now find use in a record number of homes. Indeed, the number of multiple-PC homes (households with one or more PCs) is also growing rapidly. Over the next few years, the number of multiple-PC homes is expected to grow at a double-digit rate while the growth from single-PC homes is expected to remain flat. At the same time, the popularity and pervasiveness of the well-known Internet has produced a need for faster and less expensive home-based access.
As is well known, usage of the Internet has exploded during the past few years. More and more often the Internet is the preferred medium for information exchange, correspondence, research, entertainment, and a variety of other communication needs. Not surprisingly, home-based Internet usage has also increased rapidly in recent years. A larger number of homes require access to the Internet than ever before. The increase in home Internet usage has produced demands for higher access speeds and increased Internet availability. To meet these needs, advances have been made in cable modem, digital subscriber loop (DSL), broadband wireless, powerline local loop, and satellite technologies. All of these technologies (and others) are presently being used to facilitate home-based Internet access. Due to these technological advances and due to the ever-increasing popularity of the Internet, predictions are that home-based Internet access will continue to explode during the next decade. For example, market projections for cable modem and DSL subscriptions alone show an imbedded base of approximately 35 million connected users by the year 2003.
In addition to recent technological advances in the personal computing and Internet access industries, advances have also been made with respect to appliances and other equipment intended for home use. For example, because an increasing number of people work from home, home office equipment (including telecommunication equipment) has become increasingly complex and sophisticated. Products have been developed to meet the needs of the so-called SOHO (“small office, home office”) consumer. While these SOHO products tend to be less expensive than their corporate office product counterparts, they do not lack in terms of sophistication or computing/communication power. In addition to the increasing complexity of SOHO products, home appliances have also become increasingly complex and sophisticated. These so-called “smart” appliances often use imbedded microprocessors to control their functions. Exemplary smart appliances include microwaves, refrigerators, dishwashers, washing machines, dryers, ovens, etc. Similar advances have been made in home entertainment systems and equipment such as televisions (including set-top boxes), telephones, videocassette recorders (VCRs), stereos, etc. Most of these systems and devices include sophisticated control circuitry (typically implemented using microprocessors) for programming and controlling their functions. Finally, many other home use systems such as alarm systems, irrigation systems, etc., have been developed with sophisticated control sub-components.
The advances described above in home appliance and equipment technologies have created a need for similar advancements in home communication networking technology. As home appliances and entertainment products become increasingly more complex and sophisticated, the need has arisen for facilitating the interconnection and networking of home appliances and other products used in the home.
One exemplary home-based communication system is commonly referred to as “Powerline Networking”. Powerline Networking refers to the concept of using existing residential AC power lines as a means for networking all of the appliance and products used in the home. Although the existing AC power lines were originally intended for supplying AC power only, the Powerline Networking approach anticipates also using home power lines for communication networking purposes. One such proposed powerline networking approach is shown in the block diagram of FIG. 1.
As shown in FIG. 1, the powerline network 100 includes a plurality of power line outlets 102 electrically coupled to one another via a plurality of power lines 104. As shown in FIG. 1, a number of devices and appliances are coupled to the powerline network via interconnection with the plurality of outlets 102. For example, a personal computer 106, laptop computer 108, telephone 110, facsimile machine 112, and printer 114 are networked together via electrical connection with the power lines 104 through their respective and associated power outlets 102. In addition, “smart” appliances such as a refrigerator 115, washer dryer 116, microwave 118, and oven 126 are also networked together using the powerline network 100. A “smart” television 122 is networked via electrical connection with its respective power outlet 102. Finally, as shown in FIG. 1, the powerline network can access an Internet Access Network 124 via interconnection with a modem 126 or other Internet access device.
With multiple power outlets 102 in almost every room of the modem home, the plurality of power lines 104 potentially comprises the most pervasive in-home communication network in the world. The powerline network system is available anywhere power lines exist (and therefore, for all intents and purposes, it has worldwide availability). In addition, networking of home appliances and products is potentially very simple using powerline networking systems. Due to the potential ease of connectivity and installation, the powerline networking approach will likely be very attractive to the average consumer. However, powerline networking systems present a number of difficult technical challenges. In order for powerline networking systems to gain acceptance, these challenges will need to be overcome.
To appreciate the technical challenges presented by powerline networking systems, it is helpful to first describe some of the electrical characteristics unique to home powerline networks. As is well known, home power lines were not originally designed for communicating data signals. The physical topology of the home power line wiring, the physical properties of electrical cabling used to implement power lines, the types of appliances typically connected to the power lines, and the behavioral characteristics of the current that travels on the power lines all combine to create technical obstacles to using power lines as a home communication network.
The power line wiring used within a house is typically electrically analogous to a network of transmission lines connected together in a large tree-like configuration. The power line wiring has differing terminating impedances at the end of each stub of the network. As a consequence, the transfer function of the power line transmission channel has substantial variations in gain and phase across the frequency band. Further, the transfer function between a first pair of power outlets very often differs from that between a second pair of power outlets. The transmission channel tends to be fairly constant over time. Changes in the channel typically occur only when electrical devices are plugged into or removed from the power line (or occasionally when the devices are powered on/off). When used for networking devices in a powerline communications network, the frequencies used for communication typically are well above the 60-cycle AC power line frequency. Therefore, the desired communication signal spectrum is easily separated from the power-bearing signal in a receiver connected to the powerline network.
Another important consideration in the power line environment is noise and interference. Many electrical devices create large amounts of noise on the power line. The powerline networking system must be capable of tolerating the noise and interference extent on home power lines. Some home power line interference is frequency selective. Frequency selective interference causes interference only at specific frequencies (i.e., only signals operating at specific frequencies are interfered with, all other signals experience no interference). In addition, some home power line interference is impulsive by nature. Although impulsive interference spans a broad range of frequencies, it occurs only in short time bursts. Some home power line interference is a hybrid of these two types of interference (frequency selective and impulsive). In addition to the different types of interference present on home power lines, noise is neither uniform nor symmetric across the power lines.
An important aspect of any home powerline networking system specification is the definition of the modulation protocol that is used by the powerline networking systems to efficiently transmit information between transmitters and receivers. A basic powerline networking system transmitter and receiver are now described with reference to FIGS. 2a and 2b. 
FIG. 2a shows a simplified block diagram of a basic powerline networking transmitter 30. As shown in FIG. 2a, the basic powerline networking transmitter 30 includes a data source 32, a modulation operations stage 34, and a line driver and power line coupler stage 36. The data source 32 generates either an analog or digital data signal (depending on the networking system used) and provides the data signal as input to the modulation operations stage 34. The modulation operations stage 34 inputs a modulated signal to the line driver and power line coupler stage 36. The power line coupler stage 36 outputs an amplified modulated signal to a powerline network (e.g., power lines).
FIG. 2b shows a simplified block diagram of a basic powerline networking receiver 40. As shown in FIG. 2b, the basic powerline networking receiver 40 includes a power line coupler and AGC (automatic gain control) stage 42, a demodulation operations stage 44, and a data sink 46. The power line coupler and AGC stage 42 obtains inputs from a modulated signal (not shown) provided on a powerline network and inputs the modulated signal to the demodulation operations stage 44. The demodulation operations stage 44 demodulates the modulated signal and inputs a data signal to the data sink 46. The demodulation technique used by the demodulation operations stage 44 depends upon the modulation technique used by the modulation operations stage 34.
Referring again to FIG. 2a, the modulation operations stage 34 modulates the data signal by performing a series of modulation operations to the data signal. Several different modulation techniques are well known in the digital communications art. Examples of modulation techniques include amplitude modulation (AM) and frequency modulation (FM, FSK, BPSK, QPSK, etc.). The type of modulation techniques used by the modulation operations stage 34 depends upon the operating environment of the powerline networking system.
In powerline networks, power line channels are highly frequency-selective, with both the gain and the phase of the channels varying substantially over the frequency band. Thus, single carrier modulation techniques are ill suited for powerline networks because they require complex adaptive equalizers necessary to compensate for the channel. Consequently, multi-carrier modulation (MCM) techniques are well suited for powerline networking systems.
Orthogonal Frequency Division Multiplexing (OFDM) is an example of an MCM technique that is well suited for powerline networking systems. OFDM is well suited for powerline networking environments because with multiple carriers being used, the channel is essentially flat across the entire band of each carrier. Advantageously, no equalization is required in order to recover a signal when individual carriers use differential phase modulation.
OFDM modulation techniques are well known in the modulation design art as exemplified by their description in an article entitled “Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come”, by John A. C. Bingham, published in IEEE Communications Magazine at pages 5-14, in May 1990, the text of which is hereby fully incorporated by reference herein for its teachings on data transmission and modulation techniques. Typical OFDM systems generate transmitted waveforms using Inverse Fast-Fourier Transforms (IFFT). The modulation of each carrier uses rectangular pulses, and thus, the entire OFDM time domain waveform can be created by simply setting an appropriate amplitude and phase for the points in the frequency domain (or tones) that correspond to each carrier, and by implementing the IFFT to create a time domain waveform. The time duration of this time domain waveform is equal to the inverse of the frequency spacing between the tones. The term “OFDM symbol” is typically used to denote the time domain waveform that results from a single IFFT operation. Typical OFDM systems transmit OFDM symbols sequentially (i.e., one symbol after another), and optionally transmit overhead signaling between the OFDM symbols.
One aspect of the OFDM modulation techniques is that carriers are “orthogonal”. The carriers are orthogonal because each carrier has an integer number of periods in the time interval that is generated by the IFFT. The orthogonal characteristic of OFDM modulation allows OFDM receivers to perform Fast-Fourier Transform (FFT) computations that yield the original data bits without creating intersymbol interference.
OFDM modulation techniques transmit data by dividing a data stream into several parallel bit streams. The bit-rate of each of these bit streams is much lower than the aggregate bit-rate of all of the streams. The bit streams are used to modulate several densely spaced and overlapping sub-carriers. Although the sub-carriers overlap in frequency spectrum, their orthogonal relation allows sufficient separation for demodulation purposes.
In a multiple access communication system, such as an OFDM home power line network, transmitters typically transmit only when data is queued (i.e., only when a transmitter has data to transmit). Receivers or modems used in such systems are known as “burst modems” because the communication system transmits information in data bursts (i.e., high volumes of data are transmitted during a relatively short time interval). A data burst comprises a frame structure including a preamble that precedes all other information. In typical OFDM systems, the preambles include two unique sets of OFDM symbols, wherein each set comprises two or more repeated OFDM symbols. These sets are preferably poorly correlated with each other. It is important that the burst modems rapidly detect the presence of a data burst, the correct start and stop times to be used for sampling the detected data burst, and the frame boundary of the detected data burst.
One approach at rapidly detecting the presence of data bursts uses a frequency-domain correlator to detect the preambles. The frequency-domain correlator (FDC) method relies upon the well-known observation that circularly time shifting a Discrete Fourier Transform (DFT) causes a phase change between a previously time-shifted DFT to a subsequently time-shifted DFT. The FDC method is a multiple-hypothesis technique because multiple possibilities (i.e., “hypotheses”) are calculated and compared to a theoretical result.
The FDC method detects preambles by correlating (in the frequency domain) the phase of the received OFDM symbol with the phase of the transmitted OFDM symbol. Specifically, the FDC method calculates a phase for every possible circularly time-shifted combination of the DFT. The FDC method utilizes a 256-sample DFT, and thus the method calculates 256 phases. The method detects a preamble when a strong correlation occurs between any of the 256 possible phases and a theoretical phase. The time-shifted combination (i.e., one possible timing hypothesis out of 256) that produces the strongest correlation is used to correct the timing of subsequent OFDM symbols.
Unfortunately, the FDC approach has several disadvantages. First, the method is complex, and thus requires increased processing complexity. Second, the FDC method is limited because detection must balance two opposing goals: increasing detection probability and decreasing false alarms. These are opposing goals. Although raising the detection thresholds degrades detection probability and decreases a number of false alarms, lowering the detection thresholds increases both detection probability and the number of false alarms. Third, the FDC method suffers reduced detection accuracy because it does not use all available information from the transmitted signals (e.g., magnitude information). Fourth, the FDC method has limited accuracy regarding time synchronization because timing errors are limited by the spacing between hypotheses.
Therefore, a need exists for an improved method and apparatus for preamble detection and time synchronization estimation in OFDM communication systems. Specifically, a need exists for a method and apparatus that rapidly detects preambles and determines time synchronization in OFDM communication systems using data burst transmissions. Such a method and apparatus should be implemented in a simple manner that utilizes both phase and magnitude information. In addition, the method and apparatus should be capable of providing highly accurate timing synchronization estimations without using multiple-hypothesis techniques. The present invention provides such a preamble detection and time synchronization estimation method and apparatus.